In production methods to date, the pole faces of the iron parts, i.e. the armature and the yoke, are ground such that they are as flat as possible. In the process, the flatness of the rear-side bearing is imaged on the bearing surface during grinding. Owing to machine and part tolerances, this image is concave or convex. In the case of a concave image, the inner limb sides do not come to bear, i.e. an air gap is formed which weakens the secondary flux in the magnet system. This results in a reduction in the holding force and an increase in the tendency of the magnet systems to hum.
These manufacture-related fluctuations result in considerable additional costs associated with bringing the undesired humming noises to an end by means of relatively complex repair work.